We report the formation of highly organized fibrous networks on ∼50 nm thick oligomeric films of hydrolyzed γ-(aminopropyl)triethoxysilane (γ-APS) adsorbed on Na+-containing substrates. The growth of these nanostructures is dependent on the surface segregation of Na+ ions and exposure to ambient conditions. Using an atomic force microscope (AFM), we have followed the growth characteristics of the fibrous nanostructures in a time-resolved manner. The fibers, which grow 2-dimensionally, have a uniform height of approximately 3 nm and widths varying from a few nanometers to hundreds of nanometers. The AFM further shows that each individual fiber consists of a well-ordered parallel assembly of “nanostrands” with widths of approximately 8 nm and a peak-to-valley corrugation of approximately 0.4 nm. Changes in the chemical nature of the APS films as a function of film age were characterized by X-ray photoelectron spectroscopy (XPS) and transmission infrared spectroscopy (IR). Results indicate that the amine (NH2) functionality on the surface of the film reacts with atmospheric CO2 and H2O to form n-propyl carbamate species (NHCOO-). To explain the fiber growth, we postulate that the diffusing Na+ ions participate in acid−base interactions within the film leading to the formation of small γ-APS oligomers capped with NHCOO- and NH2 functional groups. The stabilization of the NHCOO- species by Na+ ions leads to self-assembly via hydrogen bonding and electrostatic interactions.

Modulus measurements are among the most useful properties available for monitoring the cure and aging of rubbers. Historically, such measurements were done on macroscopic samples, but over the past 15 years, several penetration techniques have been and are being developed that allow quantitative estimates of modulus to be made with lateral resolutions of 100 μm or better. This review summarizes these developments and the types of unique information that can be generated on rubbery materials. A large part of the review focuses on the types of results available from a modulus profiling apparatus that has been used to study rubbers for the past 15 years. This instrument allows estimates to be made of the inverse tensile compliance (closely related to Young's tensile modulus) with a lateral resolution of around 50 to 100 μm. Several recently developed alternative methods for achieving similar spatial resolution are also described. Finally, a brief review is given of the recent attempts to measure quantitative modulus values for rubbers with even better resolution using instruments historically focused on metals and other hard materials such as nano-indenters, the atomic force microscope and the interfacial force microscope.

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Using an interfacial force microscope (IFM), we have measured the elastic nanoindentation modulus of thin films of poly[(aminopropyl)siloxane] spin-coated on Na + -containing glass and native SiO2. The films were prepared by the hydrolytic polycondensation of γ-(aminopropyl)triethoxysilane (γ-APS). The elastic moduli of 500 Å thick films deposited on Na + -containing glass and native SiO2 are 8 ( 2 and 35 ( 3 GPa, respectively. IFM data were complemented with atomic force microscopy and infrared and X-ray photoelectron spectroscopy. We propose that the significantly smaller modulus of γ-APS on glass is related to the incorporation of Na + ions from the glass into the siloxane network of the film. These incorporated Na + ions act as Lewis acids and catalyze the depolymerization of the Si-O-Si network, resulting in a less rigid polysiloxane framework and a lower elastic modulus. A clue to the films' structural and chemical difference is provided by the observation of time-dependent morphological changes for γ-APS on glass but not for γ-APS on SiO2.